A coating or coating system may include cleaning, pretreatment, and multiple organic coating layers such as primer, topcoat, and clear coat. Multiple processing steps and coating layers may be employed to form a coating system.
The annual cost of corrosion to the US economy was estimated to be $276 billion in 2002. Besides these costs, corrosion also adversely impacts safety and efficiency in a number of economic sectors such as transportation and infrastructure. Corrosion control may be achieved through material and coating selection based on accelerated laboratory corrosion and service environment testing.
Government, military, and industrial performance requirements for materials qualification often rely on pass/fail measurements of scribed flat panels exposed for a given duration in aggressive environments. However, in operational environments, alloys may fail due to other corrosion processes associated with localized corrosion, galvanic couples, and mechanical loading that may not be evaluated using current practices. Known measurement approaches do not include processes and measurements that assess coating protection properties for atmospheric corrosion that can lead to subsequent generation of environment-assisted cracking, corrosion fatigue, and fatigue.
Thus, there is a need to improve coatings and materials corrosion testing. One way to do this, recognized by the inventors, is to leverage advances in sensing and instrumentation to obtain high fidelity data on corrosion performance and degradation processes. A reliable measurement system that improves the corrosion evaluation of coatings on structural materials would be useful for example for: coating development and improvement, coating and materials selection to achieve design life and warrantee performance for specific service environments, and comparative testing for optimal coating and materials selection.
To minimize risk associated with the introduction of new materials (alloys and coatings), most industries use a staged approach for product evaluation and qualification that includes a series of laboratory studies, outdoor exposure site tests, and limited use service trials. In the specific case of a new coating, full qualification usually takes a minimum of three years to achieve acceptance and often this can take much longer. The lengthy qualification process is due to the nature and variety of degradation processes being evaluated, uncertainty in test results, and coarse performance measurements.
Current military coating evaluation and qualification methods for aircraft pretreatments, primers, and full coating systems rely on salt fog testing of scribe panels according to ASTM B117. The salt fog test is a relatively simple chamber test developed in the 1950s where components or samples are exposed to constant conditions of humidity, temperature, and salt spray. It is widely accepted that salt fog testing is most appropriate for quality assurance purposes, and efforts have been made to establish more sophisticated cyclic testing like ASTM G85 to achieve conditions that are similar to those that occur in operational environments such as for automotive products and to better simulate acidic or industrial environments.
Commonly coating performance specifications require the use of flat test coupons with scribes through the paint to the bare metal (ASTM D1654, MIL-PRF-32239, ASTM G1). In general, to meet coating performance requirements there should be no blistering, lifting, or substrate pitting, and in the case of chromate primers, no corrosion in the scribe (see, e.g., MIL-PRF-32239). Although there have been efforts to establish more realistic test panels that better approximate the components of interest, and refine methods to quantify corrosion damage, currently prescribed tests tend to be pass/fail evaluations and do not provide a rank order of performance nor any time-based information related to precursor, incubation, and growth processes of damage state progression for the various mechanisms that may be occurring.
Corrosion test practices rely heavily on operator visual or optical inspections and rating criteria that are applied at the conclusion of an accelerated test or outdoor exposure (ASTM 1654). Little to no information on the corrosion kinetics is obtained, and often knowledge of the impact of galvanic processes and the effect of localized corrosion on residual strength or environment-assisted cracking is not determined. The measurement time, uncertainty, and limited mechanistic information make it difficult to assess relative performance for product acceptance and qualification, ultimately slowing coating product development and product integration.
Furthermore, often the properties being evaluated using previous methods do not measure the damage modes or mechanisms that are relevant to the specific application. Specifically, free corrosion of the alloy or blistering of the paint film may be less important than galvanic corrosion, protection against pitting, and resulting loss of strength of the structural component. Finally, since little information on the corrosion rate or rate of change of corrosion can be elucidated, the rank order performance of materials is dependent on the test period, and significant risk exists for choosing a material with poor long-term performance.
In order to obtain the maximum benefit from sophisticated test protocols and automated accelerated corrosion test chambers, improved in situ measurements of the progression of coating system breakdown and the various forms of corrosion that are relevant to a particular use or application are needed. Prior sensor research has focused on either measuring environment or corrosion in outdoor service environments using sensors that measure only limited corrosion processes.
Accordingly, there is a significant need for an improved measurement system for judging the multiple dynamic processes associated with corrosion and coating performance to achieve a more complete solution set for coating and materials development and qualification.